Add WalkerBase

[mattalxndr]

Type of changes

• New feature
• Refactoring

Checklist

• I've run the latest black with default args on new code.
• I've updated CHANGELOG.md and CONTRIBUTORS.md where appropriate.
• I've added tests for new code.
• I accept that @PyFilesystem/maintainers may be pedantic in the code review.

Description

I've been cleaning up the huge mess my personal files have become over the years, and I needed a way to list every file in every archive across all my various drives.

To that end, I'm working on a new walker class that uses https://github.com/althonos/fs.archive to recursively list files contained in nested archives.

I noticed a TODO from @althonos, along with references to a missing WalkerBase class, and figured I'd go ahead and implement it, since I was looking around for an abstract base class anyway.

I'm keeping this open, since there are a couple of things that still need attention (and some reading into how Python implemented generics):

• fs.walk.BoundWalker is still coupled with fs.walk.Walker specifically.
• fs.base.FS: is still coupled with fs.walk.Walker specifically.